Communication devices typically communicate with one another using a transceiver that includes a transmitter section coupled to an antenna (in wireless communication devices) or a cable (in wired communication devices). The transmitter section typically includes a data modulation stage that converts raw data into baseband signals in accordance with a particular communication standard and a frequency conversion stage that mixes the baseband signals with one or more local oscillations to produce radio frequency (RF) signals. A power amplifier is employed to amplify the RF signals prior to transmission via the antenna or cable.
In both wireless and wired communication devices, a particular load impedance is expected at the output of the power amplifier. This load impedance is typically determined by the impedance of the communication device's antenna or cable and a matching network. Often, however, the load impedance varies, resulting in an impedance mismatch between the power amplifier and the load. For example, the impedance of the load may vary over the frequency range of the output signal; variation in the impedance of the load may also be introduced by the cable or antenna being disconnected during transmission.
As a result of an impedance mismatch between the power amplifier and its load, excess power from the power amplifier output fails to reach the load and must be dissipated by one or more transistors in the power amplifier. In severe impedance mismatch conditions, this dissipated power may damage or destroy the transistor(s). For example, when the cable of a wired communication device is disconnected during transmission, the output voltage of the power amplifier may become quite large, resulting in breakdown of the transistor(s). To avoid the risk of device breakdown, the output voltage of the power amplifier must be reduced in these over-voltage situations.
In conventional protection circuits, a single power detector is used to reduce the power amplifier gain when the output power exceeds a given threshold. Depending on the modulation scheme used, the absolute power detection method of the conventional approach may be too slow in responding to an over-voltage situation that occurs as a result of a change in the load impedance. For example, in modulation schemes with high crest factors (e.g., orthogonal frequency-division multiplexing and higher order quadrature amplitude modulation) a longer length of time may be necessary for the power detector to converge to an accurate power estimate. Consequently, by the time the power detector converges, the output power may have exceeded breakdown limits of devices within the power amplifier for a period of time sufficient to result in damage. Moreover, the accuracy of the conventional approach is limited, since it depends on absolute circuit parameters.
Accordingly, what is desired is a system and method that is capable of rapidly recognizing and responding to an over-voltage situation by reducing a gain of a power amplifier.
The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.